Hey, cut the kid some slack, he is probably just a kid and wants to understand the relationship between cost and aircraft size.
Yes, the cost per ton is much lower with a large aircraft, but not all markets will support a large aircraft, and those markets still require timely cargo service for perishables and other items like express mail. So for a remote location where timeliness or the availability of surface transport is an issue, an aircraft like the caravan is well suited to the task, even though the unit cost is higher.